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Old 12-09-2015, 04:36 PM #1
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Default 12v PSU with DVD diode

Hi guys,

I'm new here, searched the forum but havent seemed to find an exact answer to my question. I've seen loads of diagrams for DVD laser diodes but most of them show the input voltage at around 7-9 volts. I'm wanting to run a diode off of 12v. Can I use the same kind of circuit as this but just apply 12v?



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Old 12-09-2015, 04:46 PM #2
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Default Re: 12v PSU with DVD diode

Quote:
Originally Posted by ZorAxe View Post
Hi guys,

I'm new here, searched the forum but havent seemed to find an exact answer to my question. I've seen loads of diagrams for DVD laser diodes but most of them show the input voltage at around 7-9 volts. I'm wanting to run a diode off of 12v. Can I use the same kind of circuit as this but just apply 12v?


Should work just fine with 12V - However, you're better using an input voltage closer to the voltage requirement of the diode. LM317s are linear regulators, so they'll produce more heat when the difference between input and output is larger.

For example:

Your diode needs 3.2V@300mA (I'm just making these numbers up, they may not be the same for your diode, but they will be close for some red diodes) - you supply the driver with 5V, still at 300mA. To get down to 3.2V you need to dissipate 1.8V@300mA which is 0.54W.

Now, if you supply the driver with 12V, you need to dissipate 8.8V@300mA which is 2.64W.

You may want a small heatsink on the LM317 if you are running at 12V. I'd probably put one on there just to be safe at lower voltages too.
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Last edited by diachi; 12-09-2015 at 04:52 PM.
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Old 12-09-2015, 04:53 PM #3
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Default Re: 12v PSU with DVD diode

Yes you can, that driver circuit will adjust the voltage to the diode as needed to keep the current constant, it can take much more than 12V except that your LM317 needs a heat sink as it will produce more heat the more you are above the required voltage. The value of the resistor determines what current it will provide to the diode.

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Old 12-09-2015, 05:45 PM #4
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Default Re: 12v PSU with DVD diode

Thanks for the replies. I will be using a PC PSU so there is 5v available on the output of the PSU. I'm not very clued up in electronics, but if I use those exact valued components would it work for a DVD Write diode?
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Old 12-09-2015, 06:02 PM #5
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Default Re: 12v PSU with DVD diode

Quote:
Originally Posted by ZorAxe View Post
Thanks for the replies. I will be using a PC PSU so there is 5v available on the output of the PSU. I'm not very clued up in electronics, but if I use those exact valued components would it work for a DVD Write diode?
The current is adjustable via the 100 Ohm variable resistor, so, yes it should. Not all DVD writers use the same diode though, I'd figure out what diode you have first and see how much current it needs.

At that point, you might want to build a test load to make sure you have the right current set before you attach the diode.

All of this information is readily available, these questions have probably been asked hundreds of times. Spend some time searching with the white search bar at the bottom of the page, that'll help you understand things much better and make your life easier!
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Old 12-09-2015, 06:03 PM #6
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Default Re: 12v PSU with DVD diode

you have to take into account the 1.25V drop across the resistor that us used to set the current .

Assuming a 5V supply .

3V Diode drop + 1.25V resistor drop = 4.25V , that only leaves 0.75V over the regulator and I think you'll find thats under its minimum dropout voltage - its like 2V in most cases ( it "may" work but I thinks its cutting it close )

other wise it wont regulator the current properly if that's the case .
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Old 12-09-2015, 07:42 PM #7
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Default Re: 12v PSU with DVD diode

Quote:
Originally Posted by ionlaser555 View Post
you have to take into account the 1.25V drop across the resistor that us used to set the current .

Assuming a 5V supply .

3V Diode drop + 1.25V resistor drop = 4.25V , that only leaves 0.75V over the regulator and I think you'll find thats under its minimum dropout voltage - its like 2V in most cases ( it "may" work but I thinks its cutting it close )

other wise it wont regulator the current properly if that's the case .

What about using the 5V rail as ground and the 12V rail as V+?
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Last edited by diachi; 12-09-2015 at 08:06 PM.
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Old 12-10-2015, 03:39 AM #8
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Default Re: 12v PSU with DVD diode

Quote:
Originally Posted by diachi View Post
What about using the 5V rail as ground and the 12V rail as V+?
I didn't think so, but I tried it just now to be sure. I put 200ohm (35mA load) between 5V and 12V, and it just shuts down. ATX supplies are already unstable with light loads, so this shouldn't surprise anyone. It would work if you had a 2A load on 5V already... but if you're going to waste power, it might as well be through the lm317's heat sink.
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Old 12-10-2015, 04:43 AM #9
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Default Re: 12v PSU with DVD diode

Quote:
Originally Posted by Cyparagon View Post
I didn't think so, but I tried it just now to be sure. I put 200ohm (35mA load) between 5V and 12V, and it just shuts down. ATX supplies are already unstable with light loads, so this shouldn't surprise anyone. It would work if you had a 2A load on 5V already... but if you're going to waste power, it might as well be through the lm317's heat sink.

Thanks for testing that! Figured an ATX would get flakey about that.
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